It seems the sleep mode is part of the battery saving scheme.
https://kindle.s3.amazonaws.com/Kind...s_Guide_v3.pdf
Quote:
You can leave the Kindle User’s Guide, or any item you are reading, at any time by pressing the Home button. If you are done reading, you can put your Kindle to sleep by sliding and releasing the power button located at the top of your Kindle (you wake up Kindle the same way). Sleeping your Kindle will extend battery life and also lock all of the keys and buttons so you don’t inadvertently press one while not using your Kindle. When your Kindle is asleep, you will see an image on the screen. To turn off your Kindle completely, slide and hold the power switch for four seconds, then
release. The Kindle screen will blank and the device will turn off.
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Sleep mode with wireless on allows for software updates to happen automatically.